Molten core may explain Mercury's magnetic field

Mercury likely has a partly molten core, a new study indicates. This molten material may be generating the planet's weak magnetic field, whose existence has been a puzzle since its discovery more than 30 years ago.

Earth's magnetic field is generated by molten material in its core, but Mercury is so small that it was expected to have cooled and solidified long ago. But when NASA's Mariner 10 spacecraft flew by the Sun-scorched planet in 1974 and 1975, it detected a magnetic field.

Even though the field is just 1% as strong as Earth's, its detection was taken by some scientists as an indication that Mercury had somehow avoided having its core solidify. Others argued that the core had already solidified but that back when it was still molten, the planet's crust had become magnetised, leaving behind a "fossil" magnetic field.

Now, measurements of Mercury's fluctuating spin rate are providing new evidence that Mercury really does have molten material inside it. Jean-Luc Margot of Cornell University in New York, US, led a team that made the spin measurements.

'Disco ball'

They bounced radar signals off of Mercury using the 305-metre Arecibo radio dish in Puerto Rico and the Goldstone 34-metre dish in California, US. Observations of the returning signal were made with these two dishes, as well as the 100-metre Robert C. Byrd Green Bank Telescope in West Virginia, US.

The signals reflect off of irregularities in Mercury's surface, turning it into something of a 'disco ball,' Margot says, with patterns of reflected radio waves sweeping across the solar system. The faster Mercury spins, the faster these patterns move, so watching them allowed the scientists to monitor its rotation rate.

Mercury spins once on its axis every 59 days, but the Sun's uneven gravitational pull makes the planet's spin rate fluctuate slightly over the course of its orbit. The size of these fluctuations depend on whether Mercury is solid or liquid inside - just as eggs spin differently depending on whether they are raw or cooked, Margot says.

The researchers measured the fluctuations to be about 1 part in 10,000, which is larger than the expected rate for a completely solid planet, suggesting Mercury is partially molten inside. They suspect that the inner part of the core is solid but that its outer regions are still molten.

Distant building blocks

That would be possible if its core, thought to be mostly iron, contains some sulphur as well. "If you mix in a small amount of sulphur with iron, you can lower the melting temperature by several hundred kelvin," Margot told New Scientist.

But if Mercury does contain some sulphur, its presence presents a puzzle of its own, since sulphur could not have condensed out of the early solar nebula so close to the Sun. This suggests that some of the building blocks, or planetesimals, that coalesced to form Mercury came from farther out, where sulphur could have condensed along with iron and other elements.

"Those planetesimals colliding with each other eventually bring all these elements together and that's what you end up with to form the planet," Margot says.

Sean Solomon of the Carnegie Institution of Washington in Washington, DC, US, says this is a likely explanation for Mercury's sulphur, if it really does contain a substantial amount. "If there is sulphur in Mercury's core, then as Margot has stated, it was likely contributed by planetesimals that formed farther from the Sun than Mercury's current orbit," he told New Scientist.

Other elements

But he says a variety of other elements besides sulphur could also keep Mercury's core molten.

Messenger - short for Mercury, Surface, Space Environment, Geochemistry and Ranging - will help provide a more definitive understanding of the planet's core. The radar data give a 95% confidence that the interior must be partially molten, but a better understanding of the distribution of mass within Mercury from Messenger measurements should sharply reduce the uncertainty.

"As soon as Messenger gets there [it] will make the interpretation of our results much more precise," Margot says.

The researchers have also discovered a longer term shift in Mercury's spin rate, with a period of about 12 years. They will continue to monitor the planet's spin rate in the hopes of determining the source of this shift, which might be the result of Jupiter's gravitational pull. They also plan to study Venus to see if its spin fluctuates.

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